In recent years, for the purpose of development effectiveness of the information system and operation control cost, an operation control system (so-called a cloud system), attracts lots of attention. This system uses an information system provided by another organization, without maintaining its own information system. In the cloud system, an organization which controls the information system differs from the organization which uses the information system. Thus, it is difficult for the organization itself to make a scheme to prevent information leakage, a scheme to investigate the cause of an accident occurrence, and a scheme to prevent recurrence. As a scheme for data leakage prevention, it is necessary to maintain data confidentiality with the utilization of the encryption technology.
In a known system using the encryption technology in the server/client model, while the client deposits data in the server, information leakage of the deposited data to the server is prevented. For example, Non-patent document 1 and Non-patent document 2 disclose a search processing system for searching for the deposited encrypted data without decrypting the encrypted data, in response to a client request. This search processing system uses a probabilistic encryption method in which plaintext and ciphertext have a complicated relationship of one-to-“m”correspondence to each other. This probabilistic encryption method is more secure than a deterministic encryption method in which plaintext and ciphertext have a simple relationship of one-to-one correspondence to each other. According to the proposed technology, the data deposited in the server can securely be searched, while preventing information leakage to the server administrator.